protection priorities |
|
 |
In deciding on the degree of protection which should be offered by your electrical equipment for the environment in which you intend to operate it, you need to give attention to the following three priorities.
|
|
| |
|
|
| |
|
|
| |
You need to consider the probability of the presence of a potentially explosive gas, dust or vapour and air mixture in the area in which the equipment is to operate.
Areas where there is the likelihood of the presence of explosive gas-air/dust-air mixtures are referred to as zones. Zones are classified as shown in the table below. The higher the number in this "zonal classification", the smaller the risk of an explosion.
The atmosphere of the environment in which the equipment is to operate will have a bearing on the degree of protection against explosion that should be provided by the equipment.
|
|
| |
|
|
| |
|
Zone
0 /20
|
|
Zone 0:
An area in which an explosive atmosphere is likely to be present continuously
for long periods or frequently.
Zone 20:
Continuous presence of dust cloud |
|
|
Zone
1 /21 |
|
Zone
1:
An area in which an explosive atmosphere is likely to occur during normal
operation, occasionally.
Zone
21:
Primary grade of a dust cloud release |
|
|
Zone
2 /22
|
|
Zone
2:
An area where the occurrence of an explosive atmosphere is unlikely in
normal operation and if will be of a short duration.
Zone
22:
Secondary grade of dust cloud release |
 |
| |
|
|
| |
|
|
| |
|
|
| |
|
| |
You
need to give consideration to the quantity of
heat energy needed to produce an explosion of
the gas -air/dust-air mixtures.
You also need to give
consideration to the intensity of the explosion
that would occur if there were to be an ignition
of the vapour or gas-air/dust-air mixture in the
area in which the equipment is to operate.
Gases and vapours can
be grouped according to the energy be needed to
ignite the gas and the intensity of the explosion
produced by the gas.
A table of some of the gases and vapours listed in gas groups I, IIA, IIB and IIC is shown below.
The chemical properties of the gases or vapour that may be present in the area in which the equipment is to be operated have a bearing on the degree of protection which should be provided by the equipment. |
|
| |
|
|
| |
|
|
| |
|
|
| |
|
|
| |
|
|
| |
|
|
| |
You need to give consideration to the temperature at which the vapour, gas-air or dust-air mixture would ignite and become a self-sustaining reaction in the area in which the equipment would operate.
The
surface temperature of the electrical equipment
must be such that it never exceeds the temperature
at which the vapour, dust or gas air mixture in
the equipment environment will ignite and sustain
combustion.
A
system of temperature ratings, so called "T ratings",
has been devised to cover the range of temperatures
in which it is safe to operate the electrical
equipment in the explosive environment.
The relationship
between the T rating , the ignition temperature
of the ambient explosive gas-air and the maximum
allowable surface temperature of the electrical
equipment is given in the following table. |
|
| |
|
|
| |
|
|
| |
|
(often
referred to as T rating or temperature class) |
|
|
|
T
rating
(temperature
class) |
Ignition temperature range
of the explosive gas-air mixture
|
Maximum
permitted surface temperature of electrical
equipment |
|
|
T1 |
greater
than 450ºC |
450°C |
|
|
T2 |
between
300°C and 450°C |
300°C |
|
|
T3 |
between
200°C and 300°C |
200°C |
|
|
T4 |
between
135°C and 200°C |
135°C |
|
|
T5 |
between
100°C and 135°C |
100°C |
|
|
T6 |
between
85°C and 100°C |
85°C |
|
| |
|
|
| |
For
example, town gas has an ignition temperature
of 600°C. From the table you can see that
this places it in the T1 temperature class.
Therefore the maximum permissible surface temperature
for equipment that operates in a town gas and
air environment is 450°C. Carbon disulphide
vapour on the other hand has an ignition temperature
of only 100°C.
This
gives the vapour a T rating of T6. Therefore
the maximum permissible surface temperature
for equipment in this environment is 85°C.The
T rating for some of the gases in gas groupings
IIA, IIB and IIC is given in the table below.
|
|
| |
|
|
| |
|
Gas
group |
|
T1 |
|
T2 |
|
T3 |
|
T4 |
|
T6 |
|
|
|
IIA |
|
propane
toluene |
|
n-hexane
n-butane |
|
petrol
aviation fuel |
|
acetyl
aldehyde |
|
|
| |
|
|
|
|
|
|
IIB |
|
town
gas |
|
ethylene |
|
|
|
|
|
|
|
|
IIC |
|
hydrogen |
|
|
|
|
|
|
|
carbon
disulphide |
|
| |
|
|
| |
|
|
| |
The
minimum ignition temperature of a dust cloud
is the lowest temperature of a heated impact
plate at which the dust blown into the oven ignites
or decomposes, producing flames or an explosion.
Powders show a broad spectrum
of ignition sensitivity and the vast majority need
a very energetic ignition source.The
minimum ignition temperature is not a true constant
for a given dust cloud, but depends on the geometry
of the hot surface and the dynamics of the cloud.
The
following table shows some explosive
parameters of dust. |
|
| |
|
|
| |
|
Minimum Ignition Temperature of a Dust Cloud |
|
|
|
| |
|
Material |
|
T
cloud |
|
T
layer |
|
Ignition
Energy (mj) |
|
|
|
Flour |
|
380°C |
|
300 °C |
|
>30 |
|
|
Wood
Floor |
|
410°C |
|
200°C |
|
>100 |
|
|
Aluminium |
|
560°C |
|
270°C |
|
>5 |
|
|
Sulphur |
|
240°C |
|
250°C |
|
>10 |
|
| |
|
|
| |
« selection criteria |
|
|
